1. Field of the Invention
This invention relates to hydraulically actuated pumping devices. More particularly, it is concerned with a hydraulically actuated pumping unit that includes independent speed control for the upstroke and downstroke rates of the pump's polished rod. The pumping unit described herein is particularly well adapted for use in pumping oil from an oil well.
2. Description of the Prior Art
Oil well pumping units typically include a cam operated rocking arm coupled to a sucker rod that extends into the oil well. The pivotal motion of the rocking arm is converted to the back-and-forth pumping action of the sucker rod. A rocker arm must be of sturdy construction and generally comprises a significant dead weight which must be shifted along a path of travel during each pump cycle. Moreover, the rates at which the sucker rod is shifted along its upstroke and downstroke, when coupled to a conventional rocker arm, are both dependent upon the geometry of the rocker arm and cannot be independently varied. Also, the turnaround of the sucker rod between its downstroke and upstroke is dependent on the pivotal motion of the rocker arm, which leads to abrupt shifting of the sucker rod between the downstroke and the upstroke portions of its path of travel.
The above described characteristics of an oil well pumping unit having a conventional rocker arm lead to several disadvantages. First of all, the abrupt turnaround of the sucker rod leads to stretching of the sucker rod. As the sucker rod stretches, the pumping element attached to the sucker rod is correspondingly brought to successively lower heights within the oil well as the sucker rod is shifted to its uppermost position, and the suction stroke of the pumping element is therefore effectively reduced. Secondly, it is well known that an oil well pumping unit should be operated no faster than the recovery time of the oil well. That is to say, as oil is removed from the well, a certain time elapses before the well is again filled with oil. If the oil is pumped too fast relative to the well's recovery time, the well will run dry, and further operation of the pumping unit will waste the energy used to operate the pump, and may lead to pump unit damage. Operation of the pumping unit at a rate too slow relative to the well's recovery time will allow the well to fill with water as well as oil, and the water will accumulate within the lower portion of the well. It will be appreciated that the pump suction point is located in the lower portion of the well, and when water is accumulated in the lower portion of the well it will be removed by the well pumping unit rather than the oil.
It will therefore be appreciated that the rate at which an oil well pumping unit is operated is a critical factor in overall performance of the oil well. The pumping rate is difficult to control, however, when the rate of the pump's upstroke and downstroke are both dependent on the pivotal motion of a conventional rocker arm. This is particularly true when it is realized that the amount of oil removed from the well on each upstroke is in part dependent upon the rate at which the pumping element is shifted through its upstroke. If the upstroke rate is too slow, the amount of oil shifted per pumping unit upstroke will fall off drastically.
An oil well pumping unit that was not required to shift the dead weight of a rocker arm on each pumping cycle, and which provided for independent control of the rates at which the sucker rod was shifted through its upstroke and downstroke would be a decided advantage.